The present invention relates generally to the determination of a voltage drop across a load device, and more particularly to the determination of the voltage drop across any one of a plurality of load devices using sense leads. The present invention has particular application to the determination of the voltage drops across light signal emitters within a photoplethysmographic probe.
In photoplethysmography, light signals are transmitted through a tissue under test to non-invasively determine the level of one or more blood analytes such as oxyhemoglobin (O2Hb), deoxyhemoglobin or reduced hemoglobin (RHb), carboxyhemoglobin (COHb) or methemoglobin (MetHb). One type of photophlethysmographic device includes a probe having four light signal emitters and one detector. The probe is attachable to a patient""s appendage (e.g. finger, ear lobe, nasal septum, foot) and is connectable via a cable with a monitor unit. The light signal emitters may comprise, for example, light-emitting-diodes (LEDs) or laser diodes, that are operable to transmit light signals characterized by distinct center wavelengths xcex1xe2x89xa0xcex2xe2x89xa0xcex3xe2x89xa0xcex4 through the patient""s appendage to the detector. The monitor unit supplies drive signals via drive leads in the probe cable to the light signal emitters for turning the light signal emitters on and off as desired. The monitor unit also receives an output signal via an output lead in the cable from the detector indicative of the intensities of the transmitted light signals (light exiting the patient""s appendage is referred to as transmitted). The monitor processes the output signal from the detector and, since different analytes have unique light absorbency characteristics, determines the concentrations of various blood analytes in the patient""s blood based on the intensities of the transmitted light signals. See, e.g., U.S. Pat. No. 5,842,979.
The center wavelength of the light signal output by each light signal emitter may be dependent upon a number of factors, including the operating temperature of the emitter. As may be appreciated, the accuracy of the determination of the concentrations of various blood analytes may be improved if wavelength changes in the light signals output by the emitters are tracked and compensated for in the determination of blood analyte concentrations. One manner of tracking the wavelength changes is to monitor changes in the voltage drops across the junction of the light signal emitters. Sensitive measurement techniques must be employed to monitor the voltage drop changes across the light signal emitters since such changes are typically in the range of only 10 to 30 millivolts, whereas the voltage drops in the entire light signal emitter circuit are much higher.
The present invention provides a reduced wire count voltage-drop sense system and method wherein the voltage drop across a load device is determinable using only one sense lead regardless of the number of load devices. By permitting the voltage drop to be determined using only one sense lead, the reduced wire count voltage-drop sense system and method of the present invention achieves an advantage over a dual sense wire scheme that employs a common sense lead and at least one sense lead for each load device. The reduced wire count voltage-drop sense system and method of the present invention has particular applicability to the field of photoplethysmography wherein it is desirable to determine the voltage drops across one or more light signal emitters operable to illuminate a patient tissue site for determination of one or more blood analyte levels. By reducing the number of sense leads required, the component cost and manufacturing complexity of a photoplethysmographic probe are reduced thereby reducing overall probe cost. Reducing the number of sense leads can also beneficially improve the reliability of the photoplethysmographic probe. Although various aspects and advantages of the present invention are illustrated in the context of photoplethysmography, it will be appreciated that the system and method of the present invention may have applicability in other fields.
According to one aspect of the present invention, a reduced wire count voltage-drop sense system for use in photoplethysmography includes an input drive lead, a return drive lead and a single sense lead. The input drive lead extends between an associated input drive lead terminal and an associated light signal emitter input terminal. The return drive lead extends between a light signal emitter output terminal and a return drive lead terminal. The sense lead extends between the light signal emitter output terminal and a sense lead terminal. The voltage drop across a light signal emitter (e.g., an LED or a laser diode) connected between the light signal emitter input terminal and the light signal emitter output terminal when a drive current is applied therethrough via the input drive lead is determinable from a first voltage drop, measurable across the input drive lead terminal and the sense lead terminal, and a second voltage drop, measurable across the sense lead terminal and the return drive lead terminal. In this regard, the voltage drop across the light signal emitter is, for example, determinable by subtracting the second voltage drop from the first voltage drop.
The sense system of the present invention provides for the accurate determination of the voltage drop across the light signal emitter using only one sense lead as opposed to two sense leads in a dual sense wire scheme by recognizing that the input drive lead and the return drive may be configured to have substantially similar resistance values. In this regard, the input drive lead and the return drive lead may comprise substantially equal length wires made of the same material and having substantially equal diameters. Further, the wires comprising the input drive lead and the return drive lead may be disposed within the same sheath so that they have substantially similar temperature profiles along their length.
According to another aspect of the present invention, a reduced wire count voltage-drop sense system for use in photoplethysmography includes a plurality of input drive leads, a common return lead, and a single sense lead. Each of the input drive leads extends between a separate input drive lead terminal associated with the input drive lead and a light signal emitter input terminal associated with the input drive lead. The common return lead extends between a common light signal emitter output terminal and a common return lead terminal. The sense lead extends between the common light signal emitter output terminal and the sense lead terminal. A voltage drop across any one of a plurality of light signal emitters (e.g., an LED or a laser diode) operable to illuminate a patient tissue site and connected between separate light signal emitter input terminals and the common light signal emitter output terminal when a drive current is applied therethrough via an associated one of the input drive leads is determinable from a first voltage drop and a second voltage drop. The first voltage drop is measurable across the input drive lead terminal associated with the input drive lead associated with the light signal emitter across which the voltage drop is to be determined and the sense lead terminal. The second voltage drop is measurable across the sense lead terminal and the common return lead terminal.
When only one drive current is applied to the light signal emitters at a time, the voltage drop across such light signal emitter is, for example, determinable by subtracting the second voltage drop from the first voltage drop since the second voltage drop results only from the drive current applied through such load device. When more than one drive current is simultaneously applied through more than one of the light signal emitters, the second voltage drop results from all of the drive currents that are applied through the light signal emitters and not just the drive current through the light signal emitter across which the voltage drop is desired. In this regard, the desired voltage drop is, for example, determinable by subtracting a portion of the second voltage drop from the first voltage drop. The portion of the second voltage drop that is subtracted from the first voltage drop may be obtained in accordance with the level of the drive current applied through the light signal emitter across which the voltage drop is desired in comparison to the other drive currents. For example, if there are four equal level drive currents simultaneously applied to four light signal emitters, then one-fourth of the second voltage drop may be subtracted from the first voltage drop.
It will be appreciated that the common return lead may carry, on average, more current than any one of the individual drive leads because it serves as the common return for all of the drive currents. This may affect the temperature, and thus the resistance, of the common return lead. In this regard, in addition to configuring the input drive leads and the common return lead as substantially equal length wires of the same material within the same sheath, the common return lead may be provided with a larger cross-sectional area along its length. For example, the cross-sectional area of the wire comprising the common return lead may equal the number of input drive leads multiplied by the cross-sectional area of the wires comprising the input drive leads. It will be appreciated that when the common return lead wire has a larger diameter, it may be desirable to scale the second voltage drop accordingly before subtracting it from the first voltage drop. For example, if the common return lead has four times the cross-sectional area than each of the input drive leads, then the second voltage drop may be multiplied by a factor of four.
According to one more aspect of the present invention, a reduced wire count photoplethysmographic probe includes a plurality of light signal emitters (e.g., LEDs or laser diodes), a plurality of input drive leads, a common return drive lead, and a single sense lead. Each light signal emitter includes an input terminal and an output terminal single. The output terminals of the light signal emitters are connected in common with one another. Each input drive lead is associated with one of the light signal emitters and extends between an associated input drive lead terminal and the input terminal of its associated light signal emitter. The common return drive lead extends between the commonly connected output terminals of the light signal emitters and a common return drive lead terminal. The sense lead extends between the commonly connected output terminals of the light signal emitters and the sense lead terminal. A voltage drop across any one of the light signal emitters when it is turned on by applying a drive current therethrough via the input drive lead associated therewith is determinable from a first voltage drop and a second voltage drop. The first voltage drop is measurable across the input drive lead terminal associated with the input drive lead associated with the light signal emitter across which the voltage drop is to be determined and the sense lead terminal. The second voltage drop is measurable across the sense lead terminal and the common return drive lead terminal.
According to a further aspect of the present invention, a reduced wire count photoplethysmographic probe includes at least two pairs of light signal emitters (e.g., LEDs or laser diodes), a plurality of input drive leads, a common return drive lead, and a single sense lead. The light signal emitters in each a pair of light signal emitters are connected in a back-to-back relation with one another between an input terminal of the pair and an output terminal of the pair. The lights signal emitters in each pair are oriented such that one of the light signal emitters is forward biased and one is reverse biased when a voltage is applied across the input and output terminals of the pair. The output terminals of each pair of light signal emitters is connected in common with one another. Each input drive lead is associated with one of the pairs of light signal emitters and extends between an associated input drive lead terminal and the input terminal of its associated pair of light signal emitters. The common return drive lead extends between the commonly connected output terminals of the pairs of light signal emitters and a common return drive lead terminal. The sense lead extends between the commonly connected output terminals of the pairs of light signal emitters and a sense lead terminal. The voltage drop across any one of the pairs of light signal emitters, and hence either one of the light signal emitters in the pair, when a drive current is applied therethrough via the input drive lead associated therewith is determinable from a first voltage drop and a second voltage drop. The first voltage drop is measurable across the input drive lead terminal associated with the input drive lead associated with the pair of light signal emitters across which the voltage drop is desired and the sense lead terminal. The second voltage drop is measurable across the sense lead terminal and the common return drive lead terminal.
According to yet another aspect of the present invention, a method of determining a desired voltage drop across any one of a plurality of light signal emitters operable to illuminate a patient tissue site and having separate input terminals and commonly connected output terminals proceeds in the following manner. A first voltage drop is measured across a sense lead terminal connected by a sense lead to the commonly connected output terminals of the light signal emitters and an input drive lead terminal connected by an input drive lead to the input terminal of the light signal emitter across which the desired voltage drop is to be determined when a drive current is supplied thereto via the input drive lead. A second voltage drop is measured across the same sense lead terminal and a common return drive lead terminal connected by a common return drive lead to the commonly connected output terminals of the light signal emitters. The desired voltage drop is then determined utilizing the first voltage drop and the second voltage drop. It will be appreciated that the method of the present invention does not employ the direct measurement of the voltage drop across a pair of sense lead terminals connected by dual sense leads to the input terminal of the light signal emitter across which the voltage drop is desired and the commonly connected output terminals of the light signal emitters.
When only one drive current is applied at a time, the desired voltage drop may, for example, be determined by subtracting the second voltage drop from the first voltage drop. When multiple drive currents are simultaneously supplied to more than one of the plurality of light signal emitters, the desired voltage drop may, for example, be determined by subtracting a portion of the second voltage drop from the first voltage drop. In this regard, the second voltage drop may be apportioned in accordance with the level of each drive current that is supplied to the light signal emitters in order to obtain the portion of the second voltage drop that is subtracted from the first voltage drop. Further, when the common return drive lead has a different cross-sectional area than the input drive lead, the second voltage drop may be multiplied by the ratio of the cross-sectional area of the common return drive lead to the cross-sectional area of the input drive lead prior to subtracting the second voltage drop from the first voltage drop.
These and other aspects and advantages of the present invention will be apparent upon review of the following Detailed Description when taken in conjunction with the accompanying figures.